wallet, refactor: Remove Legacy check and error

Remove dead code due to legacy wallet removal.

src/wallet/wallet.cpp

@@ -3656,9 +3656,7 @@ util::Result<std::reference_wrapper<DescriptorScriptPubKeyMan>> CWallet::AddWall
 {
     AssertLockHeld(cs_wallet);
 
-    if (!IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
+    Assert(IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));
-        return util::Error{_("Cannot add WalletDescriptor to a non-descriptor wallet")};
-    }
 
     auto spk_man = GetDescriptorScriptPubKeyMan(desc);
     if (spk_man) {